This invention relates generally to the field of electrical stimulators, and more particularly to cardioverters and defibrillators.
The earliest cardioverters and defibrillators generated either a single burst of alternating current or a single pulse for application to the heart to cause cardioversion or defibrillation. However, the use of multiple pulses to accomplish cardioversion or defibrillation has also been extensively researched. For example, U.S. Pat. No. 3,605,754 issued to Jaros, et al., on Sep. 20, 1971 discloses an early double pulse heart defibrillator employing two capacitors which are successively discharged between a single pair of electrodes. Later, the use of multiple electrode systems, in which defibrillation pulses were delivered successively between different electrode pairs chosen from among the electrodes available was suggested. For example, U.S. Pat. No. 4,727,877 issued to Kallok on Mar. 1, 1988 and U.S. Pat. No. 4,708,145 issued to Tacker, Jr. et al., on Nov. 24, 1987, both disclose a variety of implantable, multiple electrode systems adapted for use in conjunction with a sequential pulse defibrillator, in which pulses are applied sequentially to different pairs of electrodes.
The ability to deliver sequential pulses to different pairs of electrodes is incorporated in the Medtronic implantable pacemaker/cardioverter/defibrillators presently in clinical evaluation in the United States. The pulse generation circuitry in these devices corresponds generally to that disclosed in allowed U.S. Pat. No. 5,167,427, issued to Keimel, incorporated herein by reference in its entirety. In these devices, two capacitor banks are provided which are simultaneously charged and then successively or simultaneously discharged between different pairs of electrodes.
It has also been proposed to apply biphasic pulses to individual or multiple electrode pairs, in which a first pulse is followed by a second pulse of opposite polarity. The second pulse typically has an initial amplitude equal to the trailing edge amplitude of the first pulse, but at a reversed polarity, but may have an initial amplitude greater than or equal to the initial amplitude of the first pulse. Apparatus for delivering such biphasic pulses are disclosed in U.S. Pat. No. 4,850,357 issued to Bach, Jr. on Jul. 25, 1989, U.S. Pat. No. 4,953,551 issued to Mehra et al., on Sep. 4, 1990, and in U.S. Pat. No. 4,800,883 issued Jan. 31, 1989 to Winstrom, all of which are incorporated herein by reference in their entireties.
Some of the electrode configurations and pulse regimens described in the above-cited patents have all been tested clinically, and in at least some patients, provide a benefit as compared to a monophasic pulse regimen delivered between a single pair of electrodes. Some researchers have found that the use of a biphasic pulse regimen provides a reduction in the energy required to defibrillate, when the biphasic pulse is used in either a two electrode system or a three electrode system as proposed in Mehra, with two electrodes tied together during delivery of the biphasic pulse. Some researchers have also found that sequential delivery of monophasic pulses between two different electrode pairs, as described in the above-cited Kallok and Tacker Jr., references provides a reduction in energy thresholds as compared to sequentially delivered monophasic pulses. Nonetheless, there is still a substantial desire for further reductions in overall energy thresholds associated with defibrillation, in the context of implantable defibrillators. Reduction of energy threshold allows for the use of smaller batteries or provides for increased longevity in these devices, and is an area of ongoing research activities, both within the medical community and by manufacturers of implantable cardioverters and defibrillators.